In modern integrated circuit (IC) fabrication, layers of material are applied to embedded structures previously formed on semiconductor wafers. Chemical mechanical planarization (CMP) is an abrasive process used to remove these layers and polish the surface of a wafer flat to achieve the desired structure. CMP may be performed on both oxides and metals and generally involves the use of chemical slurries applied via a polishing pad that is moved relative to the wafer (e.g., the pad may rotate circularly relative to the wafer). The resulting smooth, flat surface is necessary to maintain the photolithographic depth of focus for subsequent steps and to ensure that the metal interconnects are not deformed over contour steps.
As CMP is a chemical-mechanical process, planarization/polishing performance is impacted by the mechanical properties and slurry distribution ability of the polishing pad. Polishing slurries are formulated to create passivation layers on the surface of the wafer, which passivation is removed by the mechanical action of the pad. Higher points on the wafer surface are subject to higher local pressures while lower points are protected by the passivation and the inability of the pad to reach such points. Complicating this process is the ever more prevalent use of low-K materials in modern integrated circuits. Such materials are mechanically fragile and, therefore, require that CMP processes use low down force (i.e., low compressive forces when the wafer is held against the pad during polishing operations).
FIG. 1 illustrates the surface of post-CMP copper wafer polished with a conventional polishing pad alone. As shown, the low K dielectric material 102 is capped with a protective dielectric 104 (such as silicon dioxide, silicon nitride or silicon carbide). Copper features 106 are etched into the dielectric stack. After polishing, besides dishing and erosion, damage to the dielectric stack is seen in the form of material damage 108 or delamination 110.
Conventional polishing pads are typically made of urethanes, either in cast form and filled with micro-porous elements or from non-woven felt coated with polyurethanes. During polishing, the pad surface undergoes deformation due to polishing forces. The pad surface therefore has to be “regenerated” through a conditioning process. The conditioning process involves pressing a fine, diamond covered disc against the pad surface while the pad is rotated much like during the polishing processes. The diamonds of the conditioning disc cut through and remove the top layer of the polishing pad, thereby exposing a fresh polishing pad surface underneath.
These concepts are illustrated graphically in FIGS. 2A-2C. In particular, FIG. 2A illustrates a side cutaway view of a new polishing pad 200. Polishing pad 200 contains microelements 204, and grooves 206, much like those found in commercially available polishing pads such as the ICI000 of Rhom & Haas, Inc. FIG. 2B shows the surface 202 of polishing pad 200 after polishing. The top surface of the pad shows degradation 208, especially around the microelements 204 where the edges are degraded due to plastic or viscous flow of the bulk urethane material. FIG. 2C shows the surface 202 of the polishing pad after a conditioning process has been completed. Note the depth of grooves 206 is lower than was the case for the new pad illustrated in FIG. 2A due to material removal during conditioning.
Over multiple cycles of polishing and conditioning, it is usually the case that the overall thickness of a pad wears up to a point such that the pad needs to be replaced. It is evident to those practicing in the art that pad wear rates differ from pad to pad and may also differ from one batch of pads to another batch. Currently no quantitative method exists to determine pad wear, hence end of pad life. Instead, the end of pad life is typically based on visual inspection of the pad surface to check for remaining groove depth. In the case of an un-grooved pad, end of pad life decisions are typically based on the number of wafers polished or the time elapsed since the pad was first put in service. Because such metrics are not particularly accurate it is desirable that a consistent, quantitative means to determine “end of pad life” be implemented. That is, a method based on finite wear of the pad surface would be useful in establishing a consistent basis for pad changes.